Stereophonic (stereo) sound, in opposition to monophonic (mono) sound, is when sound comes from more than one source for a better sound quality. For example, if you're watching a car driving from your left to right on TV in a pair of stereo headphones, you will really feel its movement, while the effect won't really work on mono headphones.

- Hey look ! I bought a brand new high quality sound system. It's written STEREO on it !
- Almost everything is stereo nowadays dude.